Solid-state image-intensifier for the reproduction of images produced by radiation pulses



June 6, 1961 G. DIEMER 2,987,624

SOLID-STATE IMAGE-INTENSIFIER FOR-THE REPRODUCTION OF IMAGES PRODUCED BYRADIATION PULSES Filed April 26, 1956 IG.7 GESINJS X'FQIEZ United StatesPatent O ware Filed Apr. 26, 1956, Ser. No. 580,954 Claims priority,application Netherlands May 9, 1955 Claims. (Cl. 250-213) This inventionrelates to solid-state image-intensifiers,

in which impedance variations produced in a photo-sensitive material byincident radiation govern the emission of light of a juxtaposedelectro-luminescent material. As is shown diagrammatically in Fig. 1 ofthe drawing, such a device may for example consist of a plurality oflayers directly adjacent one another, comprising outer transparentelectrodes 1 and 5 sandwiching between them a photo-conductive layer 2,an intermediate layer 3 and an electro-luminescent layer 4. Analternating voltage V is applied to the electrodes 1 and 5, and when abeam of input rays S from the left-hand side produces an image on thephoto-sensitive layer 2, so that the electric con- 'ductivity of thematerial and hence the electrical impedance of this layer are variedlocally in accordance with the intensity of the incident radiation, thedistribution of the voltage V among the various layers will also bevaried locally to a greater or smaller extent.

The electro-luminescent layer 4 thus emits output light (L in FIG. 1) inthe pattern of the image produced on the photo-sensitive layer 2. Thusan image produced by the radiation S can be amplified in intensity,while, if the beam S is not formed by visible rays (for example X- rays)the image is not only intensified, but also rendered visible.

'If the photo-sensitive layer 2 is sensitive to the electroluminescencelight emitted by the layer 4 and this light is capable of interactingwith the photo-sensitive layer, optical feed-back occurs, which may giverise to regeneration and instability of the device. Such feed-back maybe "avoided by choosing an intermediate layer 3 such that it impedes theelectro-luminescence light radiated towards the photo-sensitive layer.In order not to disturb the pattern of the image, this intermediatelayer must have a high resistance in directions lying in its plane.

The sensitivity of a solid-state image intensifier, similar to-that ofan alternating-voltage amplifier, may be increased-by means offeed-back. If the feed-back is so small that no instability occurs, noparticular dificulties arise.

-It has been suggested to apply such a strong feed-back that the imageintensifier is very unstable. To this end the intermediate layer 3 shownin Fig. 1 is omitted and the alternating voltage V is not suppliedcontinuously but suppressed periodically. The latter measure preventsthe electro-luminescent layer from becoming and remaining, as a whole,light-emissive at a maximum value.

The invention has for its object to provide a device, in which, as inthe use referred to above, the radiation image produced on thephoto-sensitive material is formed dotwise in order of time, or else, aswith stroboscopy and with X-rays produced by alternating voltage, adevice in which the radiation image is produced as a whole periodically;in general, a device in which the image to be reproduced is formed byradiation pulses and in which feedback of the electro-luminescent lighton the photosensitive material giving rise to instability is utilized ina different and simpler manner than before.

" The image-intensifier of the invention is characterized by feedback ofthe electro-luminescent light to the photosensitivematerial to such anextent, and with a certain 1 voltage applied to the electrodes, which iscontinuously operative across these electrodes during operation, 'thatthe static characteristic curve of the image intensifier has an unstablerange between a lower and an upper branch. This range is, however,restricted such that the image intensifier adjusts itself, in theabsence of the radiation image to be reproduced, to a state which isindicated by a point on the lower branch of this characteristic curveoutside the unstable range.

The term static characteristic curve of the image intensifier is to beunderstood to mean the curve indicating, for the states of equilibrium,the relationship between the intensity of a radiation emanating from anexternal source and incident on the photo-sensitive material, i.e., thelight or radiation input, and the intensity of the electro-luminescencelight radiated in consequence thereof by the electro-luminescentmaterial, i.e., the light output. In the image intensifier shown in FIG.1 this is the relationship between the intensity of S and the associatedintensity of L.

The invention will now be described with reference to the accompanyingdrawing, in which:

FIG. 1, as indicated earlier, shows a conventional image intensifier,

'intensifier as shown in FIG. 1,

FIG. 3 illustrates the efiect of the value of the voltage at theelectrodes, other conditions being the same,

'FIG. 4 is a diagrammatical view of one embodiment of a device accordingto the invention,

FIG. 5 shows a detail of this device,

FIGS. 6 and 7 show time diagrams indicating the relationship betweenvarious radiation pulses of an image to be reproduced by a device asshown in FIG. 4 and the electro-luminescence light thus emitted.

In a solid-state image intensifier the extent of feed-back is determinedby:

(l) The fraction of the electro-luminescence light produced capable ofacting upon the photo-sensitive material. In the image intensifier shownin FIG. 1 this fraction is determined by the permeability of theintermediate layer 3 to the electro-luminescence light. It should benoted that the permeability for all wavelengths of theelectro-luminescence light need not be the same. The intermediate layermay be coloured and thus exhibit a selective absorption.

(2) The sensitivity of the photo-sensitive material to light of the samespectral composition as the fraction of the electro-luminescence lightproduced striking this material, and the extent to which such light iscapable of penetrating wholly or only partly the section of thephoto-sensitive material. In the image intensifier shown in Fig. l thethickness of the photo-sensitive layer together with the nature thereofmay therefore be an important factor.

The static characteristic curve of a solid-state image intensifier isdetermined not only by the nature and the quality of the photo-sensitiveand the electro-luminescent material, but also by the value of thevoltage at the electrodes, the frequency of this voltage and the extentof feed-back of the electro-luminescence light on the photosensitivematerial.

With respect to the latter factor FIG. 2. shows various characteristiccurves which apply to image intensifiers operating in the sameconditions and difiering only in the extent of feed-back.

In FIG. 2, as well as in FIG. 3 to be described hereinafter, theintensity S of an input radiation striking the photo-sensitive materialand emanating from an external source of radiation, this radiationcausing the initially very poor conductivity of the photo-sensitivematerial to continually increase with an increase in its intensity, isplotted on the horizontal axis, and the intensity L of the outputelectro-luminescence light is plotted on the vertical axis. The curvefound by indicating the value of 'L associated with each constant valueof S is termed herein the static characteristic curve. The expressionstatic is emploeyd herein to indicate that we are concerned here withstates of equilibrium-which may otherwise be instable-so that inertia orafter-effects inherent in the substances used are left out ofconsideration.

In general, the characteristic curve of a solid-state image intensifierhas a lower and an upper branch, connected by an ascending part, whichmay be more or less strongly curved in accordance with various factorsand which may even bend backwards, as will be explained hereinafter.

In FIG. 2 the curve 21 indicates the characteristic curve of an imageintensifier as shown in FIG. 1, in which the intermediate layer '3 iscompletely impervious to the 'elec'tro-lumine scent light, and in which,consequently, no optical feed-back occurs. The voltage at the electrodesis designated by V With the same voltage at the electrodes thecharacteristic curve of an image intensifier having a certain degree offeed-back, since the intermediate layer permits the electro-luminescentlight to pass to some extent, may be indicated by curve 22. Incontradistinct'ion to the characteristic curve 21, this curve has twopoints, i.e. A (the associated 5:5 and D (the associated S=S at which itbends back, i.e. the tangent at this point is vertical. In such a casethe image intensifier is not stable under all circumstances. With anincrease in S beyond S L increases more or less abruptly up to point Bover A, after which the upper branch BC of the curve is followed. Thispart of the curve, which is actually a region of saturation, may betermed a lit operating condition, in the sense that a relatively largeamount of output light is generated. If S is then caused to decrease, Lfollows the upper branch back to point D. At this point L also decreasesmore or less abruptly and returns to point E, vertically beneath D onthe lower branch FA of the curve 22. The .area outlined by EABDEindicated in FIG. 2 by cross-hatching, is, consequently, an unstableregion or an area of unstable conditions: the theoretical course of thecurve 22 in this area, which can be calculated, is indicated by thebroken line therein.

Since the line S=0 falls out of this unstable area, an image intensifierhaving a characteristic curve corresponding to curve 22 of FIG. 2 willadjust itself, in the absence of external radiation incident on thephoto-sensitive material, always in a manner such that theelectroluminescence light is at a minimum (lower branch of thecharacteristic curve). This may be termed the dark or substantially darkoperating condition. Under conditions otherwise the same, this isdifferent with an image intensifier in which the characteristic curvehas the shape of curve 23. Such a characteristic curve may be obtainedby employing a higher degree of optical feedback than in the imageintensifier to which applies the characteristic curve 22. This higherdegree of feedback may be realized by means of a higher permeability ofthe intermediate layer 3. (FIG. 1). Since the curve 23 has more than onepoint of intersection with the line S=0, so that this line passesthrough the unstable area, a reduction of the external radiation S tozero is no longer capable of bringing an image intensifier having such acharacteristic curve from a state indicated by a point on the upperbranch (C-H) into astateindicated by a point on the lower branch F--G (Gis the point having a vertical tangent). This can be realized only bysuppressing or reducing substantially the voltage at the electrodes.Such is the case in the aforesaid known method used with an imageintensifier having such a high degree of feed-back (complete omission ofthe intermediate layer 3).

Whereas FIG. 2 shows various characteristic curves applying to the sameelectrode voltage, but to difierent degrees of feed-back, FIG. 3 showscharacteristic curves which are found with a variation in electrodevoltage, the degree of feed-back remaining the same. We start from animage intensifier which, at an electrode voltage V has a characteristiccurve equal to curve 22 of FIG. 2. This characteristic curve is alsoindicated in FIG. 3 and designated by 30. At a lower electrode voltage Vthe same image intensifier has a curve 31, which has no points with avertical tangent and hence no unstable area. With a still lower voltage(V the characteristic curve has the shape of the curve 32, which has astill flatter course. If, however, the electrode voltage is raised to avalue V exceeding V;,, the instability will increase and thecharacteristic curve can again intersect the line S=O (curve 33).

The frequency of the voltage applied to the electrodes of the imageintensifier affects practically only the positions of the lower andupper branches of the characteristic curve. The values of S with whichthe characteristic curve has a vertical tangent vary little with thefrequency of the alternating voltage.

From the foregoing it is evident that with a solidstate imageintensifier the shape of the characteristic curve may be governed by thechoice of the degree of optical feed-back, or by the value of theelectrode voltage. This is utilized with the device according to theinvention.

FIG. 4 shows diagrammatically one embodiment of a device according tothe invention.

The image on the screen of a cathode-ray tube 40 is reproduced by meansof an optical system 41 on the photo-sensitive layer 45 of a solid-stateimage intensifier, designated as a whole by 42. This intensifier isconstituted by a transparent, flat electrode 44, a photo-sensitive layer45, an intermediate layer 46, an electroluminescent layer 47, a secondtransparent, flat electrode 48 and finally, as a base for the wholestructure, a glass plate 43. The electrode 44 may be constituted by atransparent layer of metal, for example gold, and the electrode 48 maybe constituted by a very thin layer of conductive tin oxide on the glassplate 43.

The photo-sensitive layer 45 is made mainly of cadmium sulphide, whichis activated with copper and gallium, its thickness being about severalhundred ,u..

The layer 47 is mainly made of an electro-luminescent powder, consistingof copperand aluminum-activated zinc sulphide, and ureaformaldehyde, thethickness of this layer being about 50,41

The intermediate layer 46 is of a nature such that the transmission ofthe electro-luminescence light from the layer 47 is about 0.01, i.e.only about 1% of the electroq luminescence light radiated in thedirection of the photosensitive'layer 45 can interact with this layer.The intermediate layer 46 may be made of an organic colour substancesuspended in a synthetic substance, for example aniline black, whichabsorbs the electro-luminescence light.

Instead of using an absorbing intermediate layer, use may be made of areflecting layer, if desired together with an absorbing layer betweenthe photo-sensitive layer and the electro-luminescent layer, if onlycare is taken that the desired fraction of the electro-luminescencelight (in this case about 1%) is capable of striking the photosensitivelayer. The use of a reflecting layer directly adjacent theelectro-luminescent layer, this reflecting layer being made for exampleof titanium oxide in ureaformaldehyde, has the advantage that thequantity of light emitted to the right hand, i.e. observable light, isincreased.

In order to prevent the electro-luminescence light emitted by an imagepoint of the electro-luminescent layer and passed by the intermediatelayer 46 from reacting on a larger portion of the photo-sensitive layerthan that corresponding to the associated image point,

a frame of opaque black lines is printed between these layers,preferably directly on the electro-luminescent layer. These lines, whichare designated by 50 in FIG. 5, which is a front view of the imageintensifier 42 with layers partly cut away, have a width approximatelyequal to or larger than the thickness of the layer 47, while theirintermediate space is a multiple thereof. This intermediate spacedetermines the definition of the electroluminescence image. From FIG. itis evident that the lines form a rectangular grating: this however, isnot necessary; the lines may for example be parallel wavelines, forexample sinusoidal lines, as is known with X- ray intensifying screensto suppress stray radiation.

By means of a source of alternating voltage 49 the electrodes 44 and 48of the solid-state image intensifier 42 have applied to themcontinuously an adjustable, otherwise constant alternating voltage Vwhich is about 350 v. in the present case. The frequency of this voltageis about c./s.

In accordance with the invention the degree of optical feedback and thevalue of the electrode voltage V are such that the characteristic curveof the image intensifier 42 has a shape corresponding more or less tocurves 22 and 30 in FIGS. 2 and 3. This means that in the absence of animage on the screen of the cathode-ray tube 40 the intensifier adjustsitself to a state corresponding to the lower branch of thecharacteristic curve and that no or little electro-luminescence light isproduced in the layer 47, which has been referred to as the darkoperating condition.

When an image is written on the screen of the cathoderay tube 40, forexample a television image or a radar image, each image point of thephoto-sensitive layer 45 receives, once in one image period, a more orless strong luminous pulse. Distinction may be made between the case inwhich the screen of the cathode-ray tube 40 has only a short persistenceand the case in which the screen has a long persistence, the duration ofwhich exceeds the inertia or response time of the solid-state imageintensifier 42.

This inertia is determined by the time constants of the photosensitivematerial of the layer 45 and any aftereffect of the electro-luminescentlayer.

For the case of a short persistence of the screen of the cathode-raytube, FIG. 6 shows a time diagram, from which is evident the eifect ofthe intensity of the input luminous pulse on the consequent emission ofoutput electro-luminescence light. It is assumed that an image point'ofthe photo-sensitive layer 45 is first struck by a luminous pulse 61,having an intensity exceeding only little 8; (vide FIGS. 2 and 3), andone image period later by a luminous pulse 62, having a materiallyhigher intensity. The two pulses will bring the image point of theelectro-luminescent layer associated with the corresponding image pointof the photo-sensitive layer into a lit state indicated by a point onthe upper branch of the characteristic curve. The intensity of theelectroluminescence light is at this instant in both cases L associatedwith the larger part of the upper branch. Since the intensity of theluminous pulse '61 exceeds the value S to a much smaller extent thanthat of the luminous pulse 62, the point indicating the state in whichthe image point of the electro-luminescent layer lies is much nearerpoint B for the first than for the latter pulse. Thus, when the twoluminous pulses terminate, owing to which the state of the image pointof the electroluminescent layer goes back to point D along the upperbranch of the characteristic curve, the electro-luminescence of theimage point of the electro-luminescent layer will persist shorter forthe luminous pulse 61 than for the luminous pulse 62. The return alongthe upper branch is performed with a velocity which is determined in thefirst instance by the inertia of the photo-sensitive material, but alsoby the distance between the line 8:0 and the unstable area, i.e. thedistance F-E. After the state indicated by point D has been reached, theelectroluminescence intensity decreases rapidly, as is describedhereinbefore with respect to curve 22. In FIG. 6 the dot-and-dash lines63 and 64 indicate the luminous pulses emanating from theelectro-luminescent layer associated with the luminous pulses 61 and 62respectively.

The pulses 63 and 64 consist each, properly speaking, of a large numberof successive luminous flashes, since the electro-luminescent materialdoes not continuously electro-luminesce, but luminesces once in half aperiod of the alternating voltage at the electrodes. The pulses 63 and64 shown are, in fact, the envelopes of these series of luminousflashes. It should be noted that the vertical scale for the luminouspulses on the photosensitive layer (S-scale) differs from that for theluminous pulses emanating from the electro-luminescent layer (L- scale),so that the ratio between the heights of the two pulses in FIG. 6 is nota measure for their intensity ratio.

Since, as stated above, the time (At and At: respectively in FIG. 6)during which the electro-luminescence of an image point persists is thelarger, the higher is the intensity of the input luminous pulse on thephoto-sensitive layer, the total light output of an electro-luminescencepulse is an indication of this intensity. Consequently, it is evidentthat intensification occurs, on the one hand because the intensity ofthe electro-luminescence light exceeds the intensity of the luminouspulse on the photo-sensitive layer, and on the other hand because theluminous pulse emanating from the electro-luminescent layer has a longerduration than the luminous pulse on the photo-sensitive layer, thisduration being the longer, the higher is the intensity of the incidentluminous pulse. In FIG. 6 the light content of the luminous pulses onthe photo-sensitive layer is indicated by crosshatching ascending to theright hand, and that of the electro-luminescence pulses bycross-hatching descending to the right hand.

In order to ensure that upon the occurrence of a luminous pulse on thephoto-sensitive layer the associated image point of theelectro-luminescent layer is brought into a state of maximum emission (Lthe frequency of the alternating voltage at the electrodes of thesolidstate image intensifier is chosen to be so high that during theluminous pulse on the photo-sensitive layer a plurality of periods ofthis alternating voltage are performed. With a screen of the cathode-raytube 40 having a short persistence, the duration of a luminous pulse isof the order of 10" sec. Therefore, in the example shown, the frequencyof the alternating voltage V is chosen to be 10 c./s.

If the screen of the cathode-ray tube 40 has a long persistence, theinput luminous pulses forming an image point on the photo-sensitivelayer have a shape which is indicated in the time diagram of FIG. 7 by71 for low intensity and by 72 for high intensity. Owing to thepersistence of the screen of the cathode-ray tube, the two pulses have adecay tail which is longer according as the maximum intensity of thepulse is higher. As a startingpoint it is now assumed that thepersistence of the screen of the cathode-ray tube is longer than theinertia or response time of the solid-state image intensifier. In thiscase an image point of the electro-luminescent layer which is brought bya luminous pulse on the associated image point of the photo-sensitivelayer into the lit state of maximum electro-luminescence, continues toemit light until the intensity on the photo-sensitive layer drops belowthe value S (vide curve 22). At that instant the electroluminescencedecreases strongly. For the luminous pulses 71 and 72 of FIG. 7 theassociated electro-luminescence pulses are indicated by the dot-and-dashlines 73 and 74 respectively. The termination of these pulses isdetermined practically by the points of intersection of the decaycharacteristic of the pulses 71 and 72 with the line S=S these pointsbeing indicated in FIG. 7 by P and Q respectively. The duration Ar andA1 of the electro-luminescence pulses 73 and 74, respectively, aretherefore determined by the initial intensity of the associated inputluminous pulse on the photo-sensitive layer. Since, as in the case towhich applies the time diagram of FIG. 6, the intensity of theelectro-luminescence pulse is materially higher than that of the pulseon the photosensitive layer, an intensification is obtained, while thecontrast is maintained, since the duration of the electroluminescencepulses varies with the light content of the luminous pulses on thephoto-sensitive layer.

If the screen of the cathode-ray tube 40 has a persistence of a shorterduration than the inertia of the solidstate image intensifier, theinertia of the latter determines the duration of theelectro-luminescence pulse. The device operates, in this case, inaccordance with the time diagram of FIG. 6.

It will be obvious that instead of using'a cathode-ray tube 40 with anoptical system 41, use may be made of an X-ray tube, which is fed by apulsatory voltage, for example an alternating voltage of line frequency,as a source of radiation. Also in this case the photo-sensitive layer isirradiated by pulses. It is also possible to house the image intensifierin the cathode-ray tube, the photosensitive layer being made of amaterial such as cadmium sulphide, the conductivity of which can beacted upon directly by the electron beam, i.e. without the intermedial-yof a luminescent screen.

It will be obvious that difficulties may arise if the inertia of theimage intensifier is so high that a maximum electro-luminescenceproduced by a radiation pulse on the photo-sensitive layer has not yetdecreased when the associated image point of the photo-sensitive layeris struck by a subsequent radiation pulse. The response time of theimage intensifier must therefore .be smaller than an image period.

What is claimed is: V

1. An electroluminescent device comprising an impedance-varying,radiation-responsive layer and an electroluminescent layer in juxtaposedrelationship, electrode means connected to said radiation-responsive andelectroluminescent layers in such manner that a potential may be appliedacross them in series arrangement, means mounted between theradiation-responsive and electroluminescent layers and providing a givenoptical feedback between them, whereby a predetermined proportion of thelight generated by each elemental area of the electroluminescent layerimpinges upon a corresponding elemental area of the radiation-responsivelayer, means connected to said electrode means for applying continuouslythereto a periodically-varying voltage of constant amplitude, saidoptical feedback and said voltage of constant amplitude having values atwhich the static light output-light input characteristic of the devicepossesses a first stable portion corresponding to a substantially darkoperating condition, a second stable portion corresponding to a litoperating condition, and a third unstable portion between the first andsecond portions, whereby, during operation but in the absence ofimpinging radiation, the device always remains in its dark operatingcondition, and means including a pulsing radiation image source forimpinging on said radiation-responsive layer a periodically-interruptedradiation image, whereby each elemental area of the device when struckby radiation of the radiation image is excited from its dark to its litcondition, said device having a response time shorter than an imageperiod.

2. A device as set forth in claim 1 wherein the duration of theimpinging radiation pulses exceeds the period of the applied voltage.

3. A device as set forth claim 1 wherein the optical feedback meanscomprises means for confining the optical feedback from a given point ofthe electroluminescent layer to a point of the radiation-responsivelayer corresponding thereto and initially responsible for the lightoutput from said given point.

4. A device as set forth in claim 1 wherein the source of radiationincludes a cathode-ray tube and beamscanning means.

5. A device as set forth in claim 3 wherein the feedback means comprisesan opaque grating composed of opaque grating composed of opaque lineswhose thickness is not less than that of the electroluminescent layer.

References Cited in the file of this patent UNITED STATES PATENTS2,120,916 Bitner June 14, 1938 2,525,156 Tink Oct. 10, 1950 2,805,360McNaney Sept. 3, 1957 2,858,363 Kazan Oct. 28, 1958 FOREIGN PATENTS157,101 Australia June 16, 1954 OTHER REFERENCES A Solid-State ImageIntensifier, by Orthuber et al. Journal of the Optical Society ofAmerica, volume 44, No. 4, pp- 297-299, April 1954.

Kazan: Proceedings of the IRE, vol. 43, No. 12 'December 1955, pp.1888-1897.

